Abstract: The radiation patterns of the total fields and the leaky wave fields of a 1-D infinite leaky wave antenna are calculated respectively based on the equivalent principle. The leaky wave antenna is considered to be excited by either a linear electric current source or a linear magnetic current source. The radiation patterns are derived from the equivalent electric surface currents and the equivalent magnetic surface currents respectively by spectral domain method. The radiation patterns of the total fields generated by two types of sources are calculated from reciprocity for comparison. It is demonstrated that the radiation patterns of the leaky wave fields are different based on two types of equivalent surfaces.